A four-handkerchief documentary if there ever was one, Kurt Kuenne originally decided to make this film for the unborn Zachary after his dad, Andrew Bagby, a young doctor, was murdered by Zachary’s pregnant mother.

Kuenne and Bagby had been friends since their childhood in California, when Bagby often appeared in Kuenne’s home movies, which this film draws upon.

The filmmaker also travels thousands of miles, as far away as England, interviewing members of Bagby’s extended family and many pals, including those from college and medical school.

But the film’s focus shifts dramatically while Kuenne is traveling. After fatally shooting Bagby after he broke off their engagement, Shirley Turner, also a doctor, flees to her native Canada.

Turner and her lawyers work the legal system – with the help of doctors and even, incredibly, a judge – to repeatedly delay her incarceration and extradition.

After Zachary’s birth, she battles with Andrew’s parents, David and Kathleen Bagby, who close up their house and move to Newfoundland to seek justice.

Bagby’s parents end up in the agonizing position of sharing the child with the murderer of their son, even as they seek her extradition.

The footage of the four of them together is truly heartbreaking.

Given Turner’s troubled history, it’s entirely surprising when the situation reaches an emotionally devastating climax. Bagby’s parents stay in Canada to crusade for judicial reform – and his father writes a best-selling book on the subject.

This is a story that doesn’t need schmaltzy violin music the filmmaker has added to amp up the emotionality.

And Kuenne’s tendency to overedit every shot to get in as many interviews as possible (some last seconds) gets exhausting after a while.

But it’s hard to deny that the intensely personal “Dear Zachary: A Letter to a Son About His Father” is a gut-wrenching experience.